audioflinger: remove null checks

This code has null checks with intervening unconditional dereferences of
the potentially-null variable.

It's not clear that `mOutput` can ever be null to begin with. If it can
be, we have a few dereferences to guard. :)

Caught by the static analyzer:

frameworks/av/services/audioflinger/Threads.cpp:2671:19: warning: Called
C++ object pointer is null [clang-analyzer-core.CallAndMessage]
frameworks/av/services/audioflinger/Threads.cpp:2852:9: warning: Called
C++ object pointer is null [clang-analyzer-core.CallAndMessage]

Bug: None
Test: TreeHugger

Change-Id: I456581204718390088998a1fe2514dff63f6578f